not true. im a smog technician. if we put the RPM/tachometer clamp around a spark plug and the idle is at 800 rpm, the machine will read 800 RPM. if we clamp it around the coil wire THAT GOES TO A DISTRIBUTOR the machine will read out 4800 RPM when it is actually 800. but if we let the machine know that were using the coil wire itll do the math and divide the number by the amount of cylinders. and obviously, the RPM inductive pick-up wire uses the magnetic field around the spark plug/coil wire to know the actual RPM.. and the more current that goes through a wire the bigger the magnetic field.. so the coil wire obviously has more current going through it since the machine will give a higher RPM read-out with it... it might be the same amount of voltage across the spark plug but the current is different i think.. well anyway i got shocked by a coil wire and it gives more current than spark plugs do.. correct me if im wrong.